A deformable mirror device (DMD) includes a plurality of electronically controllable mirrors. Each mirror is capable of a mechanical movement in response to an electrical signal and can reflect incident light in one of two predetermined directions corresponding to the mirror's orientation. DMDs can be used as light modulators for optical signal processing as well as for displaying images electronically. For example, a DMD having an array of tightly spaced small mirrors or pixels in the form of rows and columns can be used in a projection display. The two orientations in which each mirror or pixel reflects light are defined as the "on" and "off" state of the mirror or pixel. Therefore this particular type of DMD may be referred to hereafter as a digital micro-mirror device. Because of its high pixel density, such a DMD is capable of producing images comparable in resolution to cathode ray tube (CRT) displays, liquid crystal displays, etc. Advantageously, the process for manufacturing a DMD display is compatible with that used in the semiconductor industry. Furthermore, the mirrors can be switched "on" or "off" in micro-seconds; hence, it is capable of displaying rapidly changing images. Another advantage is that the light reflected from an "off" pixel travels along a different path and is not projected onto the image screen; hence, the display has a high contrast. Since approximately 90% of the incident light falling onto the individual "on" pixels is reflected towards the image screen, this results in a more energy efficient display compared to liquid crystal projection displays.
Currently, the individual mirrors in a conventional DMD are made of an aluminum substrate that reflects light uniformly throughout the visible spectrum; therefore, the conventional DMD is monochromatic. There are several approaches to make a colour projection display system using a DMD. Two approaches involve colour filtering the light either before or after it is reflected from the DMD. In the first approach, known also as "sequential" colour, a single monochromatic DMD is used. White light first passes through a rotating colour filter wheel having red (R), green (G) and blue (B) filters. The filtered white light then falls onto a monochromatic DMD and the light reflected from the "on" pixels forms an image on a projection screen. The light reflected from the "off" pixels travels along a different path and is absorbed by a light absorber. While this sequential colour projection display system displays only one colour at a time, an observer actually perceives a colour based on the three time-integrated primary colours. This approach has certain disadvantages: a) only one colour is displayed at any given time, that reduces the brightness of the display; b) synchronization between the rotating colour filter wheel and the DMD driver is required; and c) the use of a rotating colour filter wheel makes it difficult to reduce the overall size of the display system.
In the second approach, three monochromatic DMDs are used, one for each of the three primary colours: R, G and B. Either three colour light sources or a single white light source, divided into three primary colour beams by dichroic beam-splitters, can be used in the system. The three colour images from the three monochromatic DMDs are combined into a single image to produce the desired colour picture. The disadvantages of this system include complex chip alignment, output convergence, excessive cost and large package size of the required optical system.
A different approach for a full colour projection display system is to use a colour DMD as disclosed by W. E. Nelson in U. S. Pat. No. 5,168,406, issued on Dec. 1, 1992. The disclosed colour DMD has a colour filter on top of each aluminum mirror. Advantageously, no extra light splitting and combining optical components are required in the display. In Nelson's patent, the colour filters are implemented using intrinsic absorption in dye materials. In the manufacturing of this dye colour DMD, a dye filter mosaic or dye coating is first deposited onto a glass substrate. Next, the dye filter is subsequently transferred to a DMD chip by a sublimation process. In this process, the dye filter mosaic or coating is placed next to a DMD chip and is heated by a printing head. The dye material is vaporized and condenses on the surface of the DMD. Alternatively, dye filters may be deposited by electrically charging the individual mirrors of a DMD. A dye cloud is then introduced next to the DMD and condenses on the surface of the individual charged mirrors. Another method for manufacturing a colour filter for deformable mirror device has been disclosed by M. A. Mignardi et al. in U.S. Pat. No. 5,240,818, issued on Aug. 31, 1993. In their approach, a solution of a dye-resist mixture is spun uniformly onto a nearly completed DMD chip. The dye-resist coating in the unwanted area is then removed by photo-lithography, i.e., by exposing the chip to UV light through a mask and developing it. Different dye-resist materials can be applied in the same way to form, for example, a red, green and blue multi-colour filter array. A transparent layer is then deposited to protect the dye-resist filters.
There are several disadvantages to all the above dye or dye-resist mixture filter approaches: The stability of the dye-resist or dye filters is poor and these dye or dye-resist filters normally degrades with time. As well, their performance tends to deteriorate when exposed to heat and light sources that are present in projection display systems. Also, dye-resist and dye colours filters deposited by spinning or sublimation often have poor adhesion to the mirror substrate. In addition, the quality of the dye filters can be poor, which could result in light scattering reducing the contrast of the display. Furthermore, the processes to apply these dyes or dye-resist mixtures might not be fully compatible with current manufacturing process of the DMDs.